The present invention generally relates to color printing and the production of color prints. More specifically, this invention relates to determining the amount of color ink required to produce color prints via a method and apparatus for establishing a reference printer state using recursive tone scale matching.
The production of a color print from an original print is typically performed either by a tricolor additive process or a white light subtractive process. A tricolor additive process is based on the premise of color addition, wherein specific amounts of the primary colors red, green and blue (RGB) are combined to reproduce a desired color, and the desired color produced is the combination of those red, green and blue colors added together. In the tricolor additive process, the combination of an equal amount of red, green and blue colors produces a neutral gray or white color, whereas the complete absence of red, green and blue colors yields the color black, which is the default color in a tricolor additive process. Thus, a tricolor additive process combines specific amounts of RGB color to produce a desired color.
In contrast, a white light subtractive process operates on the premise of color absorption, wherein specific amounts of the subtractive primaries cyan, magenta and yellow (CMY) are combined to reproduce a desired color, and the desired color produced is equal to the combination of those cyan, magenta and yellow colors added together. Each of the subtractive primaries cyan, magenta and yellow is the opposite of one of the primary colors, and is equal to the color white minus its opposite primary color. This is equivalent to filtering the primary color from the color white. For instance, the color cyan is the opposite of the color red and is produced by filtering the red color component of white light, thereby yielding the residual colors blue and green. Similarly, the color magenta is the opposite of the color green and is produced by filtering the green color component from white light, thereby yielding the residual colors red and blue. Finally, the color yellow is the opposite of the color blue and is produced by filtering the blue color component from white light, thereby yielding the residual colors red and green.
In the white light subtractive process, a desired color is produced by combining CMY colors that selectively absorb specific amounts of red, green and blue from the color white. Thus, the combination of an equal amount of cyan, magenta and yellow absorbs equal amounts of red, green and blue color from the color white, respectively, yielding a neutral gray or black color. Similarly, the color produced from the complete absence of cyan, magenta and yellow is the color white, which is the default color for a white light subtractive process. When producing color prints using cyan, magenta and yellow inks, the combination of these inks to produce the color black is often not dark enough. To correct this problem, the separate color black (K) is often added to the colors cyan, magenta and yellow to create color prints. Thus, a white light subtractive process combines specific amounts of CMYK inks to produce a desired color.
Regardless of whether the printer applies a tricolor additive process using RGB colors or inks, or a white light subtractive process using CMYK colors or inks, a desired color is produced by combining different amounts, or values, of these colors or inks. The desired color produced is the sum of the color densities or luminescence of the constituent color components. Color density is the logarithm of the ratio of incident light to transmitted or reflected light, and thus measures color absorption on a logarithmic scale. For example, a color density of 2.0 means that the amount of incident light is one hundred times the amount of reflected light, i.e., that 99% of the incident light is absorbed. Luminescence is the opposite of color density, and is equal to the intensity of light per unit area of source. Measured logarithmically, a luminescence of 2.0 indicates that the amount of incident light is equal to one hundred times the amount of reflected light. Dark areas or shadows have a high color density and low luminescence, whereas light areas or highlights have a low color density and high luminescence. A spectrophotometer can determine the actual color and density of a printed color patch. CIELAB, commonly referred to as LAB, includes three measurements: L* is the measurement of lightness-darkness, a* is the measurement of greenness-redness, and b* is the measurement of yellowness-blueness.
Whenever creating a color print using RGB or CMYK inks, it is often desirable or necessary to create a color profile (e.g., an International Color Consortium or ICC profile) for the printer creating the color print. A color profile is used to translate an input color from an input media into an output color in an output media. For instance, it may be desirable to translate a color image displayed on an RGB monitor to a color print produced by a CMYK color printer. In this instance, the RGB monitor and its RGB image are the input media and input colors, respectively, and the CMYK color printer and CMYK color print are the output media and output colors, respectively. The printer""s color profile translates the RGB color values from the monitor to the corresponding CMYK color values for the printer. Thus, the color profile includes those CMYK ink values that will produce the desired printer colors corresponding to the input RGB colors from the monitor. Color profiles are not limited to RGB-CMYK translations, but can also be used for RGBxe2x80x94RGB, CMYK-RGB, and CMYKxe2x80x94CMYK media translations.
In order to create a color profile, it is necessary to create a reference printer state that establishes how much of each color of printing ink must be combined to produce a desired color. It is necessary to determine these color ink value combinations for a number of practical reasons. The primary reason is that impurities and variances within different printer inks and printer paper cause the actual ink values required to produce a desired color to deviate from the theoretical ink values required to produce a desired color. For instance, a combination of equal cyan, magenta and yellow ink values would theoretically produce a neutral gray tone, but in actuality the color IF produced may differ due to absorption variations and impurities within the CMYK inks. Thus, it is necessary to determine the actual CMYK ink values required to produce the desired colors necessary to produce a real color print.
When establishing a printer reference state, a number of features are desirable. First, it is desirable that color balance is achieved such that when a certain amount of cyan, magenta and yellow ink is applied to the substrate, a neutral or gray color is produced. It is also desirable that the printer response be similar to that of a conventional printing system like a printing press. Typically, the reason for gray balancing is due to the fact that equal amounts of CYM ink typically do not produce a gray color. Therefore, the necessary amounts of CYM ink must be calculated for each CMY inkset, paper, and device combination for the entire tonal scale. A tonal scale is the spectrum of light to dark tones for a color. The maximum amount or percentage of ink of each of colorant produces the darkest tone and the minimum amount or percentage of ink produces the lightest tone. A conventional printing system typically prints a series of patches at the quarter, mid and three-quarter tones such that the amount of cyan ink is fixed and the amounts of magenta and yellow inks are varied. The color print is made up in a grid pattern, typically 10xc3x9710, and then compared to a known gray at these three tone levels. The inking combination that achieves gray balance for this printing condition is recorded for the three tones and is used in the preparation of files upstream from the press. The value in having a gray balance condition within a printing system is that this information can then be used to successfully apply UCR (Under Cover Removal) or GCR (Gray Component Replacement) printing techniques.
Prior art methods for determining a printer reference state have a number of significant shortcomings. First, they typically require manual visual inspection of color prints in order to make a subjective determination that the desired printer response and printer reference state is achieved. This is obviously undesirable, as such a determination will vary from person to person and is subject to individual error. Second, even assuming that a manual visual inspection is not subject to error, the process used to determine the reference printer state via visual inspection is largely comprised of trial and error, thereby requiring a lengthy and imprecise process to establish a reference printer state. Thus, it is desirable that the process used to determine a reference printer state be an automatic, objective process that accurately and efficiently determines the reference printer state.
Various approaches to establishing reference printer states and color correction are disclosed in U.S. Pat. Nos. 4,092,067; 4,182,560; 4,192,605; 4,468,123; 4,873,546; 5,293,258; 5,231,504; and 5,612,903, all of which are incorporated herein by reference.
These and other deficiencies in establishing a reference printer state are addressed by the present invention, which is a method for establishing a reference printer state via recursive tone scale matching. The present invention includes the ability to determine the amount of cyan, magenta and yellow ink necessary to produce a neutral color and thereby achieve color balance with high accuracy. The present invention also includes the ability to specify and match a tone scale reproduction curve thereby enabling optimal color reproduction. The present invention further establishes the reference printer state automatically and objectively, thereby determining the printer reference state quickly and accurately. Although the present invention is described using CMYK inks and colors corresponding to white light subtractive print methods and processes, those skilled in the art will recognize that the present invention is also applicable to RGB inks and colors corresponding to color additive print methods and processes.